The color of a block added in 1.8pre is not set.
Please set it yourself by editing the .minecraft/mods/rei_minimap/blockcolor.txt.

Known Bug
Saving chunks... occures when set Threading: ture and Environment Color: True at the same time and start a game by "Play Selected World".
If either Threading or Environment Color is False, I think that probably the problem does not occur.
I understand the cause somehow, but a solution is not yet found.
Workaround: Please set either Threading or Environment Color to False.
this problem FIX is after Minecraft Coder Pack supports 1.8.

Basic tip:
Disable these both features for SMP play (by using the casual

if (!world.multiplayerWorld)

), then noone could possibly complain about.
I'm not entirely sure, but according to the updated policy you would be allowed to make such cheats...
Though one could argue that a secret-base detector (cavemap, entity radar) could be used for excessive griefing.

As i've said, just disable these both features for SMP and noone can complain ^^

Can you make it to disable mapping wood and leaves so we can see the ground?

I think you can do that manually, in \.minecraft\mods\rei_minimap\blockcolor.txt, where you just delete the line of text of the block you don't want to show up on the map. I'm not sure if it works, so it's best to make a backup of the blockcolor.txt first :tongue.gif: